The present invention relates generally to disc drive data storage systems having multiple discs mounted on a spindle to form a disc stack. More particularly, the present invention relates to a disc drive data storage system in which data is recorded on different surfaces of the disc stack at different recording densities in order to optimize the performance of the disc drive.
A typical disc drive data storage system can include multiple magnetic discs mounted for rotation on a hub or spindle. A spindle motor causes the discs to spin and the surface of the discs to pass under respective head gimbal assemblies (HGAs). The HGAs carry transducers which write information to, and read information from the disc surfaces. An actuator mechanism moves the HGAs from track to track across surfaces of the discs under control of electronic circuitry. Read and write operations are performed through a transducer which is carried in a slider body. The slider and transducer are sometimes collectively referred to as a head, and typically a single head is associated with each disc surface. The heads are selectively moved under the control of electronic circuitry to any one of multiple circular, concentric data tracks on the corresponding disc surface by an actuator device. Each slider body includes an air bearing surface (ABS). As the disc rotates the disc drags air beneath the ABS, which develops a lifting force which causes the head to lift and fly several microinches above the disc surface.
In existing disc drive systems, one of the parameters which dictates the AREAL density is the recording density, typically designated in bits per inch (BPI). Recording density is a predetermined parameter at the disc drive design stage. All disc surfaces in the disc stack are set to one standard recording density or BPI value. The maximum recording density is usually determined by the available head, disc surface and read channel capabilities. To ensure the maximum production yield and read channel margin, this recording density is usually set at a point where the drive still has sufficient read channel margin under the worst case combination of head, media (i.e., disc surface) and read channel distribution.
This current recording density scheme results in the margin available not being maximized for good head/media surface combinations, while stressing the available channel margin for the worst head/media combinations. In a multiple disc pack drive, by probability, there will virtually always be the situation where the various head/media combinations have different margins available. A single low margin head/media combination will usually result in the disc drive not meeting the desired drive error rate, even though all other head/media combinations in the disc drive may exceed the required channel margin.